Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 27: 28 March

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Formal Charge, cont.

Recall that Formal Charge (FC) = the charge an atom would have if all bonding pairs were shared equally (polar bonds don't exist in this model).

To assign Formal Charges:

  1. Draw a correct Lewis Structure.
  2. Assign both electrons of a lone pair to its associated atom.
  3. Divide all bonding pairs, giving one electron of each pair to each atom in the bond.
  4. Calculate FC = # electrons on the unbonded (elemental) atom - # electrons assigned to the bonded atom.

Examples:

Lewis Structures for Covalent Molecules-Octet Variations

Multiple Bonds & Resonance

Recall we must show an octet (or duet for Period I) in the outer-most shell (valence electrons). When this does not occur with single electron pairs (bonds) between atoms can sometimes make it happen with multiple bonds. You might find "Clark's Method" useful for determining the bonding patterns of various molecules:

Additional exercises on Lewis Structures are available in the Lewis Structure Module.

For a modern view of bonding illustrated with QuickTime movies based on quantum calculations you may enjoy the Supplement.

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© R A Paselk

Last modified 4 April 2011